Targeting CB2-GPR55 receptor heteromers modulates cancer cell signaling

Modulation of the Endocannabinoid System as a Potential Anticancer Strategy

Currently, the involvement of the endocannabinoid system in cancer development and possible options for a cancer-regressive effect of cannabinoids are controversially discussed. In recent decades, a number of preclinical studies have shown that cannabinoids have an anticarcinogenic potential. Therefore, especially against the background of several legal simplifications with regard to the clinical application of cannabinoid-based drugs, an extended basic knowledge about the complex network of the individual components of the endocannabinoid system is required. The canonical endocannabinoid system consists of the endocannabinoids N-arachidonoylethanolamine (anandamide) and 2-arachidonoylglycerol as well as the G_i/o protein-coupled transmembrane cannabinoid receptors CB₁ and CB₂. As a result of extensive studies on the broader effect of these factors, other fatty acid derivatives, transmembrane and intracellular receptors, enzymes and lipid transporters have been identified that contribute to the effect of endocannabinoids when defined in the broad sense as “extended endocannabinoid system.” Among these additional components, the endocannabinoid-degrading enzymes fatty acid amide hydrolase and monoacylglycerol lipase, lipid transport proteins of the fatty acid-binding protein family, additional cannabinoid-activated G protein-coupled receptors such as GPR55, members of the transient receptor family, and peroxisome proliferator-activated receptors were identified as targets for possible strategies to combat cancer progression. Other endocannabinoid-related fatty acids such as 2-arachidonoyl glyceryl ether, O-arachidonoylethanolamine, N-arachidonoyldopamine and oleic acid amide showed an effect via cannabinoid receptors, while other compounds such as endocannabinoid-like substances exert a permissive action on endocannabinoid effects and act via alternative intracellular target structures. This review gives an overview of the modulation of the extended endocannabinoid system using the example of anticancer cannabinoid effects, which have been described in detail in preclinical studies.

Anti-tumour actions of cannabinoids

The endocannabinoid system has emerged as an important target for the treatment of many diverse diseases. In addition to the well-established palliative effects of cannabinoids in cancer therapy, phytocannabinoids, synthetic cannabinoid compounds and inhibitors of endocannabinoid degradation have attracted attention as possible systemic anticancer drugs. Results emerging from preclinical studies suggest cannabinoids elicit effects at different levels of cancer progression, including inhibition of proliferation, neovascularization, invasion and chemoresistance, induction of apoptosis and autophagy as well as enhancement of tumour immune surveillance. Although the clinical use of cannabinoid receptor ligands is limited by their psychoactivity, non-psychoactive compounds, such as cannabidiol, have gained attention due to preclinically established anticancer properties and a favourable risk-to-benefit profile. Thus, cannabinoids may complement the currently used collection of chemotherapeutic agents, as a broadly diversified option for cancer treatment, while counteracting some of their severe side effects. LINKED ARTICLES: This article is part of a themed section on 8th European Workshop on Cannabinoid Research. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v176.10/issuetoc.

Inhibition of GPR 55 improves dysregulated immune response in experimental sepsis

Sepsis is a medical condition caused by dysregulated systemic inflammatory response against infection, resulting in high mortality. Despite intensive research over the last few decades, the results from multiple clinical trials targeting specific inflammatory mediators have been disappointing. In the present study, we investigated the role of G protein-coupled receptor GPR55 modulation on immune response in an experimental sepsis model (endotoxemia). Immune response was evaluated by analyzing leukocyte-endothelial interactions and capillary perfusion in the intestinal microcirculation using intravital microscopy. In addition, the levels of plasma inflammatory cytokines were measured. The results demonstrated that GPR55 inhibition using antagonists, CID16020046 or O-1918, significantly reduced leukocyte adherence in intestinal submucosal venules and decreased proinflammatory cytokine TNF-α and IL-6 production. These data suggest that GPR55 inhibition may be a novel therapeutic target for attenuating hyperinflammation during sepsis.

The Endocannabinoid System as a Target in Cancer Diseases: Are We There Yet?

The endocannabinoid system (ECS) has been placed in the anti-cancer spotlight in the last decade. The immense data load published on its dual role in both tumorigenesis and inhibition of tumor growth and metastatic spread has transformed the cannabinoid receptors CB1 (CB1R) and CB2 (CB2R), and other members of the endocannabinoid-like system, into attractive new targets for the treatment of various cancer subtypes. Although the clinical use of cannabinoids has been extensively documented in the palliative setting, clinical trials on their application as anti-cancer drugs are still ongoing. As drug repurposing is significantly faster and more economical than de novo introduction of a new drug into the clinic, there is hope that the existing pharmacokinetic and safety data on the ECS ligands will contribute to their successful translation into oncological healthcare. CB1R and CB2R are members of a large family of membrane proteins called G protein-coupled receptors (GPCR). GPCRs can form homodimers, heterodimers and higher order oligomers with other GPCRs or non-GPCRs. Currently, several CB1R and CB2R-containing heteromers have been reported and, in cancer cells, CB2R form heteromers with the G protein-coupled chemokine receptor CXCR4, the G protein-coupled receptor 55 (GPR55) and the tyrosine kinase receptor (TKR) human V-Erb-B2 Avian Erythroblastic Leukemia Viral Oncogene Homolog 2 (HER2). These protein complexes possess unique pharmacological and signaling properties, and their modulation might affect the antitumoral activity of the ECS. This review will explore the potential of the endocannabinoid network in the anti-cancer setting as well as the clinical and ethical pitfalls behind it, and will develop on the value of cannabinoid receptor heteromers as potential new targets for anti-cancer therapies and as prognostic biomarkers.

The onus of cannabinoids in interrupting the molecular odyssey of breast cancer: A critical perspective on UPRER and beyond

Cannabinoids, commonly used for medicinal and recreational purposes, consist of various complex hydrophobic molecules obtained from Cannabis sativa L. Acting as an inhibitory molecule; they have been investigated for their antineoplastic effect in various breast tumor models. Lately, it was found that cannabinoid treatment not only stimulates autophagy-mediated apoptotic death of tumor cells through unfolded protein response (UPRER) activated downstream effectors, but also imposes cell cycle arrest. The exploitation of UPRER tumors as such is believed to be a major molecular event and is therefore employed in understanding the development and progression of breast tumor. Simultaneously, the data on clinical trials following administration of cannabinoid is currently being explored to find its role not only in palliation but also in the treatment of breast cancer. The present study summarizes new achievements in understanding the extent of therapeutic progress and highlights recent developments in cannabinoid biology towards achieving a better cure of breast cancer through the exploitation of different cannabinoids.

GPR55 controls functional differentiation of self-renewing epithelial progenitors for salivation

GPR55, a lipid-sensing receptor, is implicated in cell cycle control, malignant cell mobilization, and tissue invasion in cancer. However, a physiological role for GPR55 is virtually unknown for any tissue type. Here, we localize GPR55 to self-renewing ductal epithelial cells and their terminally differentiated progeny in both human and mouse salivary glands. Moreover, we find GPR55 expression downregulated in salivary gland mucoepidermoid carcinomas and GPR55 reinstatement by antitumor irradiation, suggesting that GPR55 controls renegade proliferation. Indeed, GPR55 antagonism increases cell proliferation and function determination in quasiphysiological systems. In addition, Gpr55-/- mice present ~50% enlarged submandibular glands with many more granulated ducts, as well as disordered endoplasmic reticuli and with glycoprotein content. Next, we hypothesized that GPR55 could also modulate salivation and glycoprotein content by entraining differentiated excretory progeny. Accordingly, GPR55 activation facilitated glycoprotein release by itself, inducing low-amplitude Ca2+ oscillations, as well as enhancing acetylcholine-induced Ca2+ responses. Topical application of GPR55 agonists, which are ineffective in Gpr55-/- mice, into adult rodent submandibular glands increased salivation and saliva glycoprotein content. Overall, we propose that GPR55 signaling in epithelial cells ensures both the life-long renewal of ductal cells and the continuous availability of saliva and glycoproteins for oral health and food intake.

On the influence of cannabinoids on cell morphology and motility of glioblastoma cells

The mechanisms behind the anti-tumoral effects of cannabinoids by impacting the migratory activity of tumor cells are only partially understood. Previous studies demonstrated that cannabinoids altered the organization of the actin cytoskeleton in various cell types. As actin is one of the main contributors to cell motility and is postulated to be linked to tumor invasion, we tested the following hypothesizes: 1) Can cannabinoids alter cell motility in a cannabinoid receptor dependent manner? 2) Are these alterations associated with reorganizations in the actin cytoskeleton? 3) If so, what are the underlying molecular mechanisms? Three different glioblastoma cell lines were treated with specific cannabinoid receptor 1 and 2 agonists and antagonists. Afterwards, we measured changes in cell motility using live cell imaging and alterations of the actin structure in fixed cells. Additionally, the protein amount of phosphorylated p44/42 mitogen-activated protein kinase (MAPK), focal adhesion kinases (FAK) and phosphorylated FAK (pFAK) over time were measured. Cannabinoids induced changes in cell motility, morphology and actin organization in a receptor and cell line dependent manner. No significant changes were observed in the analyzed signaling molecules. Cannabinoids can principally induce changes in the actin cytoskeleton and motility of glioblastoma cell lines. Additionally, single cell motility of glioblastoma is independent of their morphology. Furthermore, the observed effects seem to be independent of p44/42 MAPK and pFAK pathways.

Therapeutic targeting of HER2-CB2R heteromers in HER2-positive breast cancer

Although human epidermal growth factor receptor 2 (HER2)-targeted therapies have dramatically improved the clinical outcome of HER2-positive breast cancer patients, innate and acquired resistance remains an important clinical challenge. New therapeutic approaches and diagnostic tools for identification, stratification, and treatment of patients at higher risk of resistance and recurrence are therefore warranted. Here, we unveil a mechanism controlling the oncogenic activity of HER2: heteromerization with the cannabinoid receptor CB₂R. We show that HER2 physically interacts with CB₂R in breast cancer cells, and that the expression of these heteromers correlates with poor patient prognosis. The cannabinoid Δ⁹-tetrahydrocannabinol (THC) disrupts HER2-CB₂R complexes by selectively binding to CB₂R, which leads to (i) the inactivation of HER2 through disruption of HER2-HER2 homodimers, and (ii) the subsequent degradation of HER2 by the proteasome via the E3 ligase c-CBL. This in turn triggers antitumor responses in vitro and in vivo. Selective targeting of CB₂R transmembrane region 5 mimicked THC effects. Together, these findings define HER2-CB₂R heteromers as new potential targets for antitumor therapies and biomarkers with prognostic value in HER2-positive breast cancer.

Receptor-Receptor Interactions as a Widespread Phenomenon: Novel Targets for Drug Development?

The discovery of receptor-receptor interactions (RRI) has expanded our understanding of the role that G protein-coupled receptors (GPCRs) play in intercellular communication. The finding that GPCRs can operate as receptor complexes, and not only as monomers, suggests that several different incoming signals could already be integrated at the plasma membrane level via direct allosteric interactions between the protomers that form the complex. Most research in this field has focused on neuronal populations and has led to the identification of a large number of RRI. However, RRI have been seen to occur not only in neurons but also in astrocytes and, outside the central nervous system, in cells of the cardiovascular and endocrine systems and in cancer cells. Furthermore, RRI involving the formation of macromolecular complexes are not limited to GPCRs, being also observed in other families of receptors. Thus, RRI appear as a widespread phenomenon and oligomerization as a common mechanism for receptor function and regulation. The discovery of these macromolecular assemblies may well have a major impact on pharmacology. Indeed, the formation of receptor complexes significantly broadens the spectrum of mechanisms available to receptors for recognition and signaling, which may be implemented through modulation of the binding sites of the adjacent protomers and of their signal transduction features. In this context, the possible appearance of novel allosteric sites in the receptor complex structure may be of particular relevance. Thus, the existence of RRI offers the possibility of new therapeutic approaches, and novel pharmacological strategies for disease treatment have already been proposed. Several challenges, however, remain. These include the accurate characterization of the role that the receptor complexes identified so far play in pathological conditions and the development of ligands specific to given receptor complexes, in order to efficiently exploit the pharmacological properties of these complexes.

Anandamide inhibits FcεRI-dependent degranulation and cytokine synthesis in mast cells through CB2 and GPR55 receptor activation. Possible involvement of CB2-GPR55 heteromers

Activation of high affinity receptor for IgE (FcεRI) by IgE/antigen complexes in mast cells (MCs) leads to the release of preformed pro-inflammatory mediators stored in granules by a Ca²⁺-dependent process known as anaphylactic degranulation. Degranulation inhibition has been proposed as a strategy to control allergies and chronic inflammation conditions. Cannabinoids are important inhibitors of inflammatory reactions but their effects on IgE/Ag-mediated MCs responses are not well described. In this study, we analyzed the effect of the endocannabinoid anandamide (AEA), the selective CB₂ receptor agonist HU308, and the GPR55 receptor agonist lysophosphatidylinositol (LPI) on FcεRI-induced activation in murine bone marrow-derived mast cells (BMMCs). Our results show that AEA, HU380 and LPI inhibited FcεRI-induced degranulation in a concentration-dependent manner. This effect was mediated by CB₂ and GPR55 receptor activation through a mechanism insensitive to pertussis toxin. Degranulation inhibition was prevented by CB₂ and GPR55 antagonism, but not by CB₁ receptor blockage. AEA also inhibited calcium-dependent cytokine mRNA synthesis induced by FcεRI crosslinking, without affecting early phosphorylation events. In addition, AEA, HU308 and LPI inhibited intracellular Ca²⁺ rise in response to IgE/Ag. CB₂ and GPR55 receptor antagonism could not prevent the inhibition produced by AEA and HU308, but partially blocked the one caused by LPI. These results indicate that AEA inhibits IgE/Ag-induced degranulation through a mechanism that includes the participation of CB₂ and GPR55 receptors acting in close crosstalk, and show that CB₂-GPR55 heteromers are important negative regulators of FcεRI-induced responses in MCs.

Modulation of the Oxidative Stress and Lipid Peroxidation by Endocannabinoids and Their Lipid Analogues

Growing evidence supports the pivotal role played by oxidative stress in tissue injury development, thus resulting in several pathologies including cardiovascular, renal, neuropsychiatric, and neurodegenerative disorders, all characterized by an altered oxidative status. Reactive oxygen and nitrogen species and lipid peroxidation-derived reactive aldehydes including acrolein, malondialdehyde, and 4-hydroxy-2-nonenal, among others, are the main responsible for cellular and tissue damages occurring in redox-dependent processes. In this scenario, a link between the endocannabinoid system (ECS) and redox homeostasis impairment appears to be crucial. Anandamide and 2-arachidonoylglycerol, the best characterized endocannabinoids, are able to modulate the activity of several antioxidant enzymes through targeting the cannabinoid receptors type 1 and 2 as well as additional receptors such as the transient receptor potential vanilloid 1, the peroxisome proliferator-activated receptor alpha, and the orphan G protein-coupled receptors 18 and 55. Moreover, the endocannabinoids lipid analogues N-acylethanolamines showed to protect cell damage and death from reactive aldehydes-induced oxidative stress by restoring the intracellular oxidants-antioxidants balance. In this review, we will provide a better understanding of the main mechanisms triggered by the cross-talk between the oxidative stress and the ECS, focusing also on the enzymatic and non-enzymatic antioxidants as scavengers of reactive aldehydes and their toxic bioactive adducts.

Molecular and functional interaction between GPR18 and cannabinoid CB2 G-protein-coupled receptors. Relevance in neurodegenerative diseases

GPR18, still considered an orphan receptor, may respond to endocannabinoids, whose canonical receptors are CB₁ and CB₂. GPR18 and CB₂ receptors share a role in peripheral immune response regulation and are co-expressed in microglia, which are immunocompetent cells in the central nervous system (CNS). We aimed at identifying heteroreceptor complexes formed by GPR18 and CB₁R or CB₂R in resting and activated microglia. Receptor-receptor interaction was assessed using energy-transfer approaches, and receptor function by determining cAMP levels and ERK1/2 phosphorylation in heterologous cells and primary cultures of microglia. Heteroreceptor identification in primary cultures of microglia was achieved by in situ proximity ligation assays. Energy transfer results showed interaction of GPR18 with CB₂R but not with CB₁R. CB₂-GPR18 heteroreceptor complexes displayed particular functional properties (heteromer prints) often consisting of negative cross-talk (activation of one receptor reduces signaling arising from the partner receptor) and cross-antagonism (the response of one of the receptors is blocked by a selective antagonist of the partner receptor). Activated microglia showed the heteromer print (negative cross-talk and bidirectional cross-antagonism) and increased expression of CB₂R and GPR18. Due to the important role of CB₂R in neuroprotection, we further investigated heteroreceptor occurrence in primary cultures of microglia from transgenic mice overexpressing human APP_Sw,Ind, an Alzheimer's disease model. Microglial cells from transgenic mice showed the heteromer print and functional interactions that were similar to those found in cells from wild-type animals that were activated by treatment with lipopolysaccharide and interferon-γ. Our results suggest that GPR18 and its heteromers may play important roles in neurodegenerative processes.

Lysophosphatidylinositols, from Cell Membrane Constituents to GPR55 Ligands

Lysophosphatidylinositols (LPIs) are membrane constituents that alter the properties of said membranes. However, recent data showing that the once orphan receptor, GPR55, can act as a receptor for LPIs has sparked a renewed interest in LPIs as bioactive lipids. As evidence supporting the importance of LPIs and/or GPR55 is continuously accumulating and because LPI levels are altered in a number of pathologies such as obesity and cancer, the coming years should bring new, exciting discoveries to this field. In this review, we discuss the recent work on LPIs and on their molecular target, the GPR55 receptor. First, we summarize the metabolism of LPIs before outlining the cellular pathways activated by GPR55. Then, we review the actions of LPIs and GPR55 that could have potential pharmacological or therapeutic applications in several pathophysiological settings, such as cancer, obesity, pain, and inflammation.

CB2 cannabinoid receptor activation promotes colon cancer progression via AKT/GSK3β signaling pathway

The pharmacological activation of the cannabinoid receptor type 2, CB2, has been shown to elicit anti-tumoral mechanisms in different cancer types. However, little is known about its endogenous role in tumor pathophysiology, and different studies have attributed pro-tumorigenic properties to this receptor. In a previous work, we showed that CB2 expression is a poor prognostic factor in colon cancer patients. Here we report that activation of CB2 with low doses of specific agonists induce cell proliferation and favor the acquisition of aggressive molecular features in colon cancer cells. We show that sub-micromolar concentrations of CB2-specific agonists, JWH-133 and HU-308, promote an increase in cell proliferation rate through the activation of AKT/PKB pathway in colon cancer in vitro and in vivo. AKT activation promotes GSK3β inhibition and thus, a more aggressive cell phenotype with the subsequent elevation of SNAIL levels, E-cadherin degradation and β-catenin delocalization from cell membrane. Taken together, our data show that CB2 activation with sub-micromolar doses of agonists, which could be more similar to endogenous levels of cannabinoids, promote colon cancer progression, implicating that CB2 could have a pro-tumorigenic endogenous role in colon cancer.

Targeting the endocannabinoid system as a potential anticancer approach

The endocannabinoid system is currently under intense investigation due to the therapeutic potential of cannabinoid-based drugs as treatment options for a broad variety of diseases including cancer. Besides the canonical endocannabinoid system that includes the cannabinoid receptors CB₁ and CB₂ and the endocannabinoids N-arachidonoylethanolamine (anandamide) and 2-arachidonoylglycerol, recent investigations suggest that other fatty acid derivatives, receptors, enzymes, and lipid transporters likewise orchestrate this system as components of the endocannabinoid system when defined as an extended signaling network. As such, fatty acids acting at cannabinoid receptors (e.g. 2-arachidonoyl glyceryl ether [noladin ether], N-arachidonoyldopamine) as well as endocannabinoid-like substances that do not elicit cannabinoid receptor activation (e.g. N-palmitoylethanolamine, N-oleoylethanolamine) have raised interest as anticancerogenic substances. Furthermore, the endocannabinoid-degrading enzymes fatty acid amide hydrolase and monoacylglycerol lipase, lipid transport proteins of the fatty acid binding protein family, additional cannabinoid-activated G protein-coupled receptors, members of the transient receptor potential family as well as peroxisome proliferator-activated receptors have been considered as targets of antitumoral cannabinoid activity. Therefore, this review focused on the antitumorigenic effects induced upon modulation of this extended endocannabinoid network.

Trends in GPCR drug discovery: new agents, targets and indications

G protein-coupled receptors (GPCRs) are the most intensively studied drug targets, mostly due to their substantial involvement in human pathophysiology and their pharmacological tractability. Here, we report an up-to-date analysis of all GPCR drugs and agents in clinical trials, which reveals current trends across molecule types, drug targets and therapeutic indications, including showing that 475 drugs (~34% of all drugs approved by the US Food and Drug Administration (FDA)) act at 108 unique GPCRs. Approximately 321 agents are currently in clinical trials, of which ~20% target 66 potentially novel GPCR targets without an approved drug, and the number of biological drugs, allosteric modulators and biased agonists has increased. The major disease indications for GPCR modulators show a shift towards diabetes, obesity and Alzheimer disease, although several central nervous system disorders are also highly represented. The 224 (56%) non-olfactory GPCRs that have not yet been explored in clinical trials have broad untapped therapeutic potential, particularly in genetic and immune system disorders. Finally, we provide an interactive online resource to analyse and infer trends in GPCR drug discovery.

An Update on Non-CB1, Non-CB2 Cannabinoid Related G-Protein-Coupled Receptors

The endocannabinoid system (ECS) has been shown to be of great importance in the regulation of numerous physiological and pathological processes. To date, two Class A G-protein-coupled receptors (GPCRs) have been discovered and validated as the main therapeutic targets of this system: the cannabinoid receptor type 1 (CB1), which is the most abundant neuromodulatory receptor in the brain, and the cannabinoid receptor type 2 (CB2), predominantly found in the immune system among other organs and tissues. Endogenous cannabinoid receptor ligands (endocannabinoids) and the enzymes involved in their synthesis, cell uptake, and degradation have also been identified as part of the ECS. However, its complex pharmacology suggests that other GPCRs may also play physiologically relevant roles in this therapeutically promising system. In the last years, GPCRs such as GPR18 and GPR55 have emerged as possible missing members of the cannabinoid family. This categorization still stimulates strong debate due to the lack of pharmacological tools to validate it. Because of their close phylogenetic relationship, the Class A orphan GPCRs, GPR3, GPR6, and GPR12, have also been associated with the cannabinoids. Moreover, certain endo-, phyto-, and synthetic cannabinoid ligands have displayed activity at other well-established GPCRs, including the opioid, adenosine, serotonin, and dopamine receptor families. In addition, the cannabinoid receptors have also been shown to form dimers with other GPCRs triggering cross-talk signaling under specific conditions. In this mini review, we aim to provide insight into the non-CB1, non-CB2 cannabinoid-related GPCRs that have been reported thus far. We consider the physiological relevance of these molecular targets in modulating the ECS.

Endocannabinoid Analytical Methodologies: Techniques That Drive Discoveries That Drive Techniques

Identification of the two major endogenous cannabinoid ligands, known as endocannabinoids, N-arachidonoyl-ethanolamine (anandamide, AEA) and 2-arachidonoyl-glycerol (2-AG), opened the way for the identification and isolation of other lipid congeners, all derivatives of fatty acids and related to the Endocannabinoid System. The nomenclature of this anandamide-type class of lipids is evolving as new species are discovered all the time. However, they each fall under the larger umbrella of lipids that are a conjugation of a fatty acid with an amine through and amide bond, which we will refer to as lipoamines. Specific subspecies of lipoamines that have been discovered are the N-acyl-ethanolamides (including AEA), N-acyl-dopamines, N-acyl-serotonins, N-acyl-GABA, N-acyl-taurines, and a growing number of N-acyl amino acids. Emerging data from multiple labs also show that monoacylglycerols (including 2-AG), COX-2 metabolites, and fatty acid esters of hydroxyl fatty acids are interconnected with these lipoamines at both the biosynthetic and metabolic levels. Understanding the molecular relatedness of these lipids is important for studying how they act as signaling molecules; however, a first step in this process hinges on advances in being able to accurately measure them.

Cannabinoid Receptor-Related Orphan G Protein-Coupled Receptors

Of the druggable group of G protein-coupled receptors in the human genome, a number remain which have yet to be paired with an endogenous ligand-orphan GPCRs. Among these 100 or so entities, 3 have been linked to the cannabinoid system. GPR18, GPR55, and GPR119 exhibit limited sequence homology with the established CB₁ and CB₂ cannabinoid receptors. However, the pharmacology of these orphan receptors displays overlap with CB₁ and CB₂ receptors, particularly for GPR18 and GPR55. The linking of GPR119 to the cannabinoid receptors is less convincing and emanates from structural similarities of endogenous ligands active at these GPCRs, but which do not cross-react. This review describes the evidence for describing these orphan GPCRs as cannabinoid receptor-like receptors.

Emerging therapeutic targets in cancer induced bone disease: A focus on the peripheral type 2 cannabinoid receptor

Skeletal complications are a common cause of morbidity in patients with primary bone cancer and bone metastases. The type 2 cannabinoid (Cnr2) receptor is implicated in cancer, bone metabolism and pain perception. Emerging data have uncovered the role of Cnr2 in the regulation of tumour-bone cell interactions and suggest that agents that target Cnr2 in the skeleton have potential efficacy in the reduction of skeletal complications associated with cancer. This review aims to provide an overview of findings relating to the role of Cnr2 receptor in the regulation of skeletal tumour growth, osteolysis and bone pain, and highlights the many unanswered questions and unmet needs. This review argues that development and testing of peripherally-acting, tumour-, Cnr2-selective ligands in preclinical models of metastatic cancer will pave the way for future research that will advance our knowledge about the basic mechanism(s) by which the endocannabinoid system regulate cancer metastasis, stimulate the development of a safer cannabis-based therapy for the treatment of cancer and provide policy makers with powerful tools to assess the science and therapeutic potential of cannabinoid-based therapy. Thus, offering the prospect of identifying selective Cnr2 ligands, as novel, alternative to cannabis herbal extracts for the treatment of advanced cancer patients.

Preclinical and Clinical Assessment of Cannabinoids as Anti-Cancer Agents

Cancer is the second leading cause of death in the United States with 1.7 million new cases estimated to be diagnosed in 2016. This disease remains a formidable clinical challenge and represents a substantial financial burden to the US health care system. Therefore, research and development of novel therapeutics for the treatment of cancer is of high priority. Cannabinoids and their derivatives have been utilized for their medicinal and therapeutic properties throughout history. Cannabinoid activity is regulated by the endocannabinoid system (ECS), which is comprised of cannabinoid receptors, transporters, and enzymes involved in cannabinoid synthesis and breakdown. More recently, cannabinoids have gained special attention for their role in cancer cell proliferation and death. However, many studies investigated these effects using in vitro models which may not adequately mimic tumor growth and metastasis. As such, this article aims to review study results which evaluated effects of cannabinoids from plant, synthetic and endogenous origins on cancer development in preclinical animal models and to examine the current standing of cannabinoids that are being tested in human cancer patients.

From Phytocannabinoids to Cannabinoid Receptors and Endocannabinoids: Pleiotropic Physiological and Pathological Roles Through Complex Pharmacology

Apart from having been used and misused for at least four millennia for, among others, recreational and medicinal purposes, the cannabis plant and its most peculiar chemical components, the plant cannabinoids (phytocannabinoids), have the merit to have led humanity to discover one of the most intriguing and pleiotropic endogenous signaling systems, the endocannabinoid system (ECS). This review article aims to describe and critically discuss, in the most comprehensive possible manner, the multifaceted aspects of 1) the pharmacology and potential impact on mammalian physiology of all major phytocannabinoids, and not only of the most famous one Δ9-tetrahydrocannabinol, and 2) the adaptive pro-homeostatic physiological, or maladaptive pathological, roles of the ECS in mammalian cells, tissues, and organs. In doing so, we have respected the chronological order of the milestones of the millennial route from medicinal/recreational cannabis to the ECS and beyond, as it is now clear that some of the early steps in this long path, which were originally neglected, are becoming important again. The emerging picture is rather complex, but still supports the belief that more important discoveries on human physiology, and new therapies, might come in the future from new knowledge in this field.

Molecular signatures of mu opioid receptor and somatostatin receptor 2 in pancreatic cancer

Pancreatic ductal adenocarcinoma (PDAC), a particularly aggressive malignancy, has been linked to atypical levels, certain mutations, and aberrant signaling of G-protein-coupled receptors (GPCRs). GPCRs have been challenging to target in cancer because they organize into complex networks in tumor cells. To dissect such networks with nanometer-scale precision, here we combine traditional biochemical approaches with superresolution microscopy methods. A novel interaction specific to PDAC is identified between mu opioid receptor (MOR) and somatostatin receptor 2 (SSTR2). Although MOR and SSTR2 did not colocalize in healthy pancreatic cells or matching healthy patient tissues, the pair did significantly colocalize in pancreatic cancer cells, multicellular tumor spheroids, and cancerous patient tissues. Moreover, this association in pancreatic cancer cells correlated with functional cross-talk and increased metastatic potential of cells. Coactivation of MOR and SSTR2 in PDAC cells led to increased expression of mesenchymal markers and decreased expression of an epithelial marker. Together these results suggest that the MOR-SSTR2 heteromer may constitute a novel therapeutic target for PDAC.

Cannabinoid receptor 2 (CB2) agonists and antagonists: a patent update

INTRODUCTION

Modulation of the CB2 receptor is an interesting approach for pain and inflammation, arthritis, addictions, neuroprotection, and cancer, among other possible therapeutic applications, and is devoid of central side effects.

AREAS COVERED

This review highlights the novel scaffolds for CB2 ligands and the diverse therapeutic applications for CB2 modulators disclosed in patents published since 2012.

EXPERT OPINION

Structural diversity of CB2 modulator scaffolds characterized the patent literature. Several CB2 agonists reached clinical Phase II for pain management and inflammation. Other therapeutic applications need to be explored such as neuroprotection and/or neurodegeneration.

Antitumorigenic targets of cannabinoids - current status and implications

INTRODUCTION

Molecular structures of the endocannabinoid system have gained interest as potential pharmacotherapeutical targets for systemic cancer treatment.

AREAS COVERED

The present review covers the contribution of the endocannabinoid system to cancer progression. Particular focus will be set on the accumulating preclinical data concerning antimetastatic, anti-invasive and anti-angiogenic mechanisms induced by cannabinoids.

EXPERT OPINION

The main goal of targeting endocannabinoid structures for systemic anticancer treatment is the comparatively good safety profile of cannabinoid compounds. In addition, antitumorigenic mechanisms of cannabinoids are not restricted to a single molecular cascade but involve multiple effects on various levels of cancer progression such as angiogenesis and metastasis. Particularly the latter effect has gained interest for pharmacological interventions. Thus, drugs aiming at the endocannabinoid system may represent potential 'antimetastatics' for an upgrade of a future armamentarium against cancer diseases.

GPR55 - a putative "type 3" cannabinoid receptor in inflammation

G protein-coupled receptor 55 (GPR55) shares numerous cannabinoid ligands with CB1 and CB2 receptors despite low homology with those classical cannabinoid receptors. The pharmacology of GPR55 is not yet fully elucidated; however, GPR55 utilizes a different signaling system and downstream cascade associated with the receptor. Therefore, GPR55 has emerged as a putative "type 3" cannabinoid receptor, establishing a novel class of cannabinoid receptor. Furthermore, the recent evidence of GPR55-CB1 and GPR55-CB2 heteromerization along with its broad distribution from central nervous system to peripheries suggests the importance of GPR55 in various cellular processes and pathologies and as a potential therapeutic target in inflammation.

Cannabinoid receptor CB1 regulates STAT3 activity and its expression dictates the responsiveness to SR141716 treatment in human glioma patients' cells

Herein we show that a majority of human brain tumor samples and cell lines over-expressed cannabinoid receptor CB1 as compared to normal human astrocytes (NHA), while uniformly expressed low levels of CB2. This finding prompted us to investigate the therapeutic exploitation of CB1 inactivation by SR141716 treatment, with regard to its direct and indirect cell-mediated effects against gliomas. Functional studies, using U251MG glioma cells and primary tumor cell lines derived from glioma patients expressing different levels of CB1, highlighted SR141716 efficacy in inducing apoptosis via G1 phase stasis and block of TGF-β1 secretion through a mechanism that involves STAT3 inhibition. According to the multivariate role of STAT3 in the immune escape too, interestingly SR141716 lead also to the functional and selective expression of MICA/B on the surface of responsive malignant glioma cells, but not on NHA. This makes SR141716 treated-glioma cells potent targets for allogeneic NK cell-mediated recognition through a NKG2D restricted mechanism, thus priming them for NK cell antitumor reactivity. These results indicate that CB1 and STAT3 participate in a new oncogenic network in the complex biology of glioma and their expression levels in patients dictate the efficacy of the CB1 antagonist SR141716 in multimodal glioma destruction.

The use of cannabinoids as anticancer agents

It is well-established that cannabinoids exert palliative effects on some cancer-associated symptoms. In addition evidences obtained during the last fifteen years support that these compounds can reduce tumor growth in animal models of cancer. Cannabinoids have been shown to activate an ER-stress related pathway that leads to the stimulation of autophagy-mediated cancer cell death. In addition, cannabinoids inhibit tumor angiogenesis and decrease cancer cell migration. The mechanisms of resistance to cannabinoid anticancer action as well as the possible strategies to develop cannabinoid-based combinational therapies to fight cancer have also started to be explored. In this review we will summarize these observations (that have already helped to set the bases for the development of the first clinical studies to investigate the potential clinical benefit of using cannabinoids in anticancer therapies) and will discuss the possible future avenues of research in this area.

The GPR55 antagonist CID16020046 protects against intestinal inflammation

BACKGROUND

G protein-coupled receptor 55 (GPR55) is a lysophospholipid receptor responsive to certain cannabinoids. The role of GPR55 in inflammatory processes of the gut is largely unknown. Using the recently characterized GPR55 inhibitor CID16020046, we determined the role of GPR55 in experimental intestinal inflammation and explored possible mechanisms of action.

METHODS

Colitis was induced by either 2.5% dextran sulfate sodium (DSS) supplemented in the drinking water of C57BL/6 mice or by a single intrarectal application of trinitrobenzene sulfonic acid (TNBS).

KEY RESULTS

Daily application of CID16020046 (20 mg/kg) significantly reduced inflammation scores and myeloperoxidase (MPO) activity. In the DSS colitis model, levels of tumor necrosis factor alpha (TNF-α) and interleukin 1 beta (IL-1β), and the expression of cyclooxygenase (Cox)-2 and signal transducer and activator of transcription 3 (STAT-3) were reduced in colon tissues while in TNBS-induced colitis, levels of Cox-2, IL-1β and IL-6 were significantly lowered. Evaluation of leukocyte recruitment by flow cytometry indicated reduced presence of lymphocytes and macrophages in the colon following GPR55 inhibition in DSS-induced colitis. In J774A.1 mouse macrophages, inhibition of GPR55 revealed reduced migration of macrophages and decreased CD11b expression, suggesting that direct effects of CID16020046 on macrophages may have contributed to the improvement of colitis. GPR55(-/-) knockout mice showed reduced inflammation scores as compared to wild type mice in the DSS model suggesting a pro-inflammatory role in intestinal inflammation.

CONCLUSIONS & INFERENCES

Pharmacological blockade of GPR55 reduces experimental intestinal inflammation by reducing leukocyte migration and activation, in particular that of macrophages. Therefore, CID16020046 represents a possible drug for the treatment of bowel inflammation.

Anandamide, Acting via CB2 Receptors, Alleviates LPS-Induced Neuroinflammation in Rat Primary Microglial Cultures

Microglial activation is a polarized process divided into potentially neuroprotective phenotype M2 and neurotoxic phenotype M1, predominant during chronic neuroinflammation. Endocannabinoid system provides an attractive target to control the balance between microglial phenotypes. Anandamide as an immune modulator in the central nervous system acts via not only cannabinoid receptors (CB1 and CB2) but also other targets (e.g., GPR18/GPR55). We studied the effect of anandamide on lipopolysaccharide-induced changes in rat primary microglial cultures. Microglial activation was assessed based on nitric oxide (NO) production. Analysis of mRNA was conducted for M1 and M2 phenotype markers possibly affected by the treatment. Our results showed that lipopolysaccharide-induced NO release in microglia was significantly attenuated, with concomitant downregulation of M1 phenotypic markers, after pretreatment with anandamide. This effect was not sensitive to CB1 or GPR18/GPR55 antagonism. Administration of CB2 antagonist partially abolished the effects of anandamide on microglia. Interestingly, administration of a GPR18/GPR55 antagonist by itself suppressed NO release. In summary, we showed that the endocannabinoid system plays a crucial role in the management of neuroinflammation by dampening the activation of an M1 phenotype. This effect was primarily controlled by the CB2 receptor, although functional cross talk with GPR18/GPR55 may occur.

In vitro and in vivo evaluation of Δ⁹-tetrahidrocannabinol/PLGA nanoparticles for cancer chemotherapy

Nanoplatforms can optimize the efficacy and safety of chemotherapy, and thus cancer therapy. However, new approaches are encouraged in developing new nanomedicines against malignant cells. In this work, a reproducible methodology is described to prepare Δ(9)-tetrahidrocannabinol (Δ(9)-THC)-loaded poly(d,l-lactide-co-glycolide) (PLGA) nanoparticles against lung cancer. The nanoformulation is further improved by surface functionalization with the biodegradable polymers chitosan and poly(ethylene glycol) (PEG) in order to optimize the biological fate and antitumor effect. Mean nanoparticle size (≈ 290 nm) increased upon coating with PEG, CS, and PEG-CS up to ≈ 590 nm, ≈ 745 nm, and ≈ 790 nm, respectively. Surface electrical charge was controlled by the type of polymeric coating onto the PLGA particles. Drug entrapment efficiencies (≈ 95%) were not affected by any of the polymeric coatings. On the opposite, the characteristic sustained (biphasic) Δ(9)-THC release from the particles can be accelerated or slowed down when using PEG or chitosan, respectively. Blood compatibility studies demonstrated the adequate in vivo safety margin of all of the PLGA-based nanoformulations, while protein adsorption investigations postulated the protective role of PEGylation against opsonization and plasma clearance. Cell viability studies comparing the activity of the nanoformulations against human A-549 and murine LL2 lung adenocarcinoma cells, and human embryo lung fibroblastic MRC-5 cells revealed a statistically significant selective cytotoxic effect toward the lung cancer cell lines. In addition, cytotoxicity assays in A-549 cells demonstrated the more intense anticancer activity of Δ(9)-THC-loaded PEGylated PLGA nanoparticles. These promising results were confirmed by in vivo studies in LL2 lung tumor-bearing immunocompetent C57BL/6 mice.

G-protein-coupled receptor type A heteromers as an emerging therapeutic target

INTRODUCTION

The discovery of receptor-receptor interactions (RRIs) in the early 1980s provided evidence that G-protein-coupled receptors (GPCRs) operate not only as monomers but also as heteromers, in which integration of the incoming signals takes place already at the plasma membrane level through allosteric RRIs. These integrative mechanisms give sophisticated dynamics to the structure and function of these receptor assemblies in terms of modulation of recognition, G-protein signaling and selectivity and switching to β-arrestin signaling.

AREAS COVERED

The present review briefly describes the concept of direct RRI and the available data on the mechanisms of oligomer formation. Further, pharmacological data concerning the best characterized heteromers involving type A GPCRs will be analyzed to evaluate their profile as possible targets for the treatment of various diseases, in particular of impacting diseases of the CNS.

EXPERT OPINION

GPCR heteromers have the potential to open a completely new field for pharmacology with likely a major impact in molecular medicine. Novel pharmacological strategies for the treatment of several pathologies have already been proposed. However, several challenges still exist to accurately characterize the role of the identified heteroreceptor complexes in pathology and to develop heteromer-specific ligands capable of efficiently exploiting their pharmacological features.